Free-cutting steel and method of producing same

ABSTRACT

Provided is a free-cutting steel that, despites not containing Pb, has machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel. A free-cutting steel comprises: a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.010% and 0.050% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0, and a steel microstructure in which at least 500 particles/mm2 of sulfide of less than 1 μm in equivalent circle diameter and at least 2000 particles/mm2 of sulfide of 1 μm to 5 μm in equivalent circle diameter are distributed.

TECHNICAL FIELD

The present disclosure relates to a free-cutting steel, in particular a steel that is a substitute for a free-cutting steel containing sulfur and a small amount of lead as elements for improving machinability by cutting. The present disclosure relates to a free-cutting steel having machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel, and a method of producing the same.

BACKGROUND

Low carbon sulfur-lead free-cutting steel as represented by JIS SUM24L contains a large amount of lead (Pb) and sulfur (S) as free-cutting elements and thus has excellent machinability by cutting.

In steel materials, lead is useful for reducing tool wear and improving chip treatability in cutting work. Hence, lead is regarded as an important element that significantly improves the machinability by cutting of materials, and is used in many steel products produced by cutting work. With the rise of environmental awareness in recent years, however, there is a growing movement to abolish or restrict the use of environmentally hazardous substances worldwide. Lead is one of such environmentally hazardous substances, and restriction on the use of lead is required.

In view of this, for example, JP H9-25539 A (PTL 1) discloses a non-Pb-containing free-cutting non-heat-treated steel. Likewise, J P 2000-160284 A (PTL 2) discloses a non-Pb-containing free-cutting steel. Moreover, JP H2-6824 B (PTL 3) discloses a free-cutting steel containing Cr which can form a compound with S more easily than Mn to thereby cause a Mn—Cr—S-based inclusion to be present and ensure machinability by cutting.

CITATION LIST Patent Literature

-   PTL 1: JP H9-25539 A -   PTL 2: JP 2000-160284 A -   PTL 3: JP H2-6824 B

SUMMARY Technical Problem

The technique described in PTL 1 is intended for a non-heat-treated steel that contains 0.2% or more of C and thus is hard, and the use of Nd which is a special element requires high production costs. With the technique described in PTL 2, adding a large amount of S causes low hot ductility and induces cracking during continuous casting or hot rolling, which is problematic in terms of surface characteristics. With the technique described in PTL 3, Cr and S are added while reducing the amount of Mn. However, due to high Cr content of 3.5% or more, not only cost reduction is difficult but also a large amount of CrS forms, causing a production problem in that material smelting treatment in the steelmaking process is difficult.

It could therefore be helpful to provide a free-cutting steel that, despites not containing Pb, has machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel and does not need to contain Nd or a large amount of S or Cr as in PTL 1 to PTL 3, together with a method of producing the same.

Solution to Problem

Upon Careful Examination, we Discovered the Following:

(i) Adding appropriate amounts of Mn, Cr, and S and optimizing the ratio 2(Mn+2Cr)/S causes an appropriate amount of sulfide to have a Mn—Cr—S composite-based composition. The sulfides of the composite-based composition can be refined by hot working.

(ii) When the sulfides are finer, the lubricating action is greater, and the formation of hard phase adhering to the tool surface, called a built-up edge, can be prevented. Thus, machinability by cutting including chip treatability and surface roughness can be significantly improved.

(iii) It is conventionally known that machinability by cutting is improved with an increase in S content in steel. There is, however, an upper limit to the amount of S that can be added in steel, from the viewpoint of hot workability or mechanical property anisotropy. If sulfides in steel are fine, machinability by cutting including chip treatability and surface roughness is significantly improved. Hence, by finely distributing sulfides in steel, favorable machinability by cutting can be ensured within the upper limit of the S content imposed from the viewpoint of hot workability or mechanical property anisotropy.

The Present Disclosure is Based on these Discoveries. We Thus Provide:

1. A free-cutting steel comprising: a chemical composition that contains (consists of), in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.0100% and 0.0500% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,

A value=2([Mn]+2[Cr])/[S]  (1)

where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S; and a steel microstructure in which at least 500 particles/mm² of sulfide of less than 1 μm in equivalent circle diameter and at least 2000 particles/mm² of sulfide of 1 μm to 5 μm in equivalent circle diameter are distributed.

2. The free-cutting steel according to 1., wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Si: 0.50% or less, P: 0.10% or less, Al: 0.010% or less, and N: 0.0150% or less.

3. The free-cutting steel according to 1. or 2., wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

4. A method of producing a free-cutting steel, the method comprising: rolling a rectangular cast steel at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, the rectangular cast steel having a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.0100% and 0.0500% or less, and Cr: 0.50% to 1.50% with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,

A value=2([Mn]+2[Cr])/[S]  (1)

where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S, and a side length of a cross section of the rectangular cast steel perpendicular to a longitudinal direction being 250 mm or more; and hot working the billet at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.

5. The method of producing a free-cutting steel according to 4., wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Si: 0.50% or less, P: 0.10% or less, Al: 0.010% or less, and N: 0.0150% or less.

6. The method of producing a free-cutting steel according to 4. or 5., wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

Advantageous Effect

It is thus possible to obtain a free-cutting steel having excellent machinability by cutting without adding lead.

DETAILED DESCRIPTION

A free-cutting steel according to the present disclosure will be described in detail below. First, the reasons for limiting the content of each component in the chemical composition of the free-cutting steel will be described below. Herein, “%” with regard to components is mass % unless otherwise stated.

C: less than 0.09%

C is an important element that greatly influences the strength and the machinability by cutting of the steel. If the C content is 0.09% or more, the steel hardens and the strength increases excessively, and as a result the machinability by cutting degrades. The C content is therefore less than 0.09%. The C content is preferably 0.07% or less. From the viewpoint of ensuring the strength, the C content is preferably 0.01% or more, and more preferably 0.03% or more.

Mn: 0.50% to 1.50%

Mn is a sulfide forming element important for improvement in machinability by cutting. If the Mn content is less than 0.50%, the amount of sulfide is small, and sufficient machinability by cutting cannot be obtained. The lower limit is therefore 0.50%. The Mn content is preferably 0.70% or more. If the Mn content is more than 1.50%, sulfides not only coarsen but also extend long, causing a decrease in machinability by cutting. In addition, the mechanical properties decrease. The upper limit of the Mn content is therefore 1.50%. The Mn content is preferably 1.20% or less.

S: 0.250% to 0.600%

S is a sulfide forming element effective in improving the machinability by cutting. If the S content is less than 0.250%, fine sulfides are few, so that the machinability by cutting cannot be improved. If the S content is more than 0.600%, sulfides coarsen excessively and the number of fine sulfides decreases, as a result of which the machinability by cutting decreases. Moreover, the hot workability and the ductility which is an important mechanical property decrease. The S content is therefore in a range of 0.250% to 0.600%. The S content is preferably 0.300% or more. The S content is preferably 0.450% or less.

O: more than 0.0100% and 0.0500% or less

O is an element that forms oxide and serves as a sulfide precipitation nucleus and also is effective in suppressing extension of sulfides during hot working such as rolling. This action can improve the machinability by cutting. If the O content is 0.0100% or less, the sulfide extension suppressing effect is insufficient and extended sulfides remain, so that the foregoing effect cannot be expected. The O content is therefore more than 0.0100%. If the O content is more than 0.0500%, not only the sulfide extension suppressing effect is saturated but also the amount of hard oxide-based inclusions increases. Adding an excessive amount of O is also economically disadvantageous. The upper limit of the O content is therefore 0.0500%.

Cr: 0.50% to 1.50%

Cr has an effect of forming sulfides and improving the machinability by cutting through lubricating action during cutting. Cr also suppresses extension of sulfides during hot working such as rolling, and thus can improve the machinability by cutting. If the Cr content is less than 0.50%, the formation of sulfides is insufficient and extended sulfides tend to remain, so that the foregoing effect cannot be expected. If the Cr content is more than 1.50%, not only the steel hardens but also sulfides coarsen. Moreover, the extension suppressing effect is saturated, and the machinability by cutting decreases. Besides, adding an excessive amount causes an increase in alloy costs, which is economically disadvantageous. The Cr content is therefore 0.50% to 1.50%. The Cr content is preferably 0.70% or more. The Cr content is preferably 1.30% or less.

The free-cutting steel contains the above-described components with the balance consisting of Fe and inevitable impurities, or contains the above-described components and further contains the below-described optional components. The free-cutting steel preferably contains the above-described components or preferably contains the above-described components and further the below-described optional components, with the balance consisting of Fe and inevitable impurities.

It is important that, in the above-described chemical composition, a A value defined by the following formula (1) is 6.0 to 18.0.

A value=2([Mn]+2[Cr])/[S]  (1)

where [M] is the content (mass %) of the corresponding element in brackets.

The A value is an important index that influences refinement of Mn—Cr—S-based sulfide during hot working such as rolling, and limiting the A value can improve the machinability by cutting. If the A value is less than 6.0, sulfide of Mn—S alone forms, which tends to be coarse. Consequently, the machinability by cutting degrades. If the A value is more than 18.0, not only the sulfide refining effect is saturated but also the amount of the sulfide forming elements is excessively large relative to sulfur, causing sulfides to coarsen. The A value is therefore 6.0 to 18.0. The A value is preferably 6.5 or more. The A value is preferably 17.0 or less.

The optional components will be described below. In addition to the above-described basic components, the free-cutting steel according to the present disclosure may optionally contain one or more selected from the group consisting of

Si: 0.50% or less,

P: 0.10% or less,

Al: 0.010% or less, and

N: 0.0150% or less.

Si: 0.50% or less

Si is a deoxidizing element. Moreover, Si oxide acts as a sulfide formation nucleus to promote the formation of sulfides and refine the sulfides and thus improve the cutting tool life. Accordingly, Si may be contained in the steel in order to further extend the tool life. If the Si content is more than 0.50%, the oxide increases in size and decreases in number. Such oxide is ineffective as a sulfide formation nucleus, and also hard oxide induces abrasive wear and leads to degradation in tool life. The Si content is therefore 0.50% or less. The Si content is preferably 0.03% or less. To achieve the foregoing action by Si, the Si content is preferably 0.001% or more.

P: 0.10% or less

P is an element effective in suppressing the formation of built-up edges during cutting work to thus reduce finishing surface roughness. From this viewpoint, the P content is preferably 0.01% or more. If the P content is more than 0.10%, the material hardens, so that the machinability by cutting decreases and the hot workability and the ductility decrease significantly. The P content is therefore preferably 0.10% or less. The P content is more preferably 0.08% or less.

Al: 0.010% or less

Al is a deoxidizing element as with Si, and may be contained in the steel. Al forms Al₂O₃ in the steel. This oxide is hard and causes degradation in cutting tool life due to abrasive wear. Hence, adding an excessive amount of Al needs to be avoided. From this viewpoint, the Al content is preferably 0.010% or less. The Al content is more preferably 0.005% or less. From the viewpoint of achieving the deoxidizing effect by Al, the Al content is preferably 0.001% or more.

N: 0.0150% or less

N forms nitride with Cr and the like. As a result of the nitride decomposing due to temperature increase during cutting work, an oxide film called belag forms on the tool surface. Belag has an action of protecting the tool surface and thereby improving the tool life. Accordingly, N may be contained in the steel. To effectively achieve this action, the N content is preferably 0.0050% or more. If the N content is more than 0.0150%, not only the effect of belag is saturated but also the material hardens, as a result of which the tool life shortens. The N content is therefore preferably 0.0150% or less. The N content is more preferably 0.0060% or more. The N content is more preferably 0.0120% or less.

The free-cutting steel according to the present disclosure may optionally further contain one or more selected from the group consisting of

Ca: 0.0010% or less,

Se: 0.30% or less,

Te: 0.15% or less,

Bi: 0.20% or less,

Sn: 0.020% or less,

Sb: 0.025% or less,

B: 0.010% or less,

Cu: 0.50% or less,

Ni: 0.50% or less,

Ti: 0.100% or less,

V: 0.20% or less,

Zr: 0.050% or less, and

Mg: 0.0050% or less.

Ca, Se, Te, Bi, Sn, Sb, B, Cu, Ni, Ti, V, Zr, and Mg each have an action of improving the machinability by cutting, and accordingly may be added in the case where the machinability by cutting is considered important. In the case of adding these elements in order to improve the machinability by cutting, if their respective contents are Ca: less than 0.0001%, Se: less than 0.02%, Te: less than 0.10%, Bi: less than 0.02%, Sn: less than 0.003%, Sb: less than 0.003%, B: less than 0.003%, Cu: less than 0.05%, Ni: less than 0.50%, Ti: less than 0.003%, V: less than 0.005%, Zr: less than 0.005%, and Mg: less than 0.0005%, sufficient effect cannot be achieved. Accordingly, their respective contents are preferably Ca: 0.0001% or more, Se: 0.02% or more, Te: 0.10% or more, Bi: 0.02% or more, Sn: 0.003% or more, Sb: 0.003% or more, B: 0.003% or more, Cu: 0.05% or more, Ni: 0.05% or more, Ti: 0.003 or more, V: 0.005% or more, Zr: 0.005% or more, and Mg: 0.0005% or more.

If their respective contents are Ca: more than 0.0010%, Se: more than 0.30%, Te: more than 0.15%, Bi: more than 0.20%, Sn: more than 0.020%, Sb: more than 0.025%, B: more than 0.010%, Cu: more than 0.50%, Ni: more than 0.50%, Ti: more than 0.100%, V: more than 0.20%, Zr: more than 0.050%, and Mg: more than 0.0050%, the effect is saturated, and also adding such amounts is economically disadvantageous. Accordingly, their respective contents are preferably Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.

(Steel Microstructure)

Distribution of at Least 500 Particles/Mm² of Sulfide of Less than 1 μm in Equivalent Circle Diameter and at Least 2000 Particles/Mm² of Sulfide of 1 μm to 5 μm in Equivalent Circle Diameter

Fine dispersion of sulfides in the microstructure of the free-cutting steel is advantageous in promoting the lubricating action between the tool and the work material during cutting work. To ensure the machinability by cutting of the free-cutting steel by such fine dispersion of sulfides, at least a predetermined amount of sulfides of less than 1 μm in equivalent circle diameter and at least a predetermined amount of sulfides of 1 μm to 5 μm in equivalent circle diameter need to be dispersed in the steel microstructure. Sulfides of less than 1 μm in equivalent circle diameter are mainly effective for lubrication between the tool and the work material. Sulfides of 1 μm to 5 μm in equivalent circle diameter not only have the foregoing lubrication effect but also are effective for chip partibility. Hence, the number of sulfides of less than 1 μm in equivalent circle diameter is at least 500 particles/mm², and the number of sulfides of 1 μm to 5 μm in equivalent circle diameter is at least 2000 particles/mm².

The conditions for producing the free-cutting steel according to the present disclosure will be described below.

A rectangular cast steel that has the above-described chemical composition and whose side length of a cross section perpendicular to the longitudinal direction is 250 mm or more is rolled at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, and the billet is hot worked at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.

(Cast Steel)

Rectangular Cross Section Whose Side Length of Cross Section Perpendicular to Longitudinal Direction is 250 mm

First, a molten steel adjusted to the chemical composition is cast to obtain a cast steel. As the cast steel, a rectangular cast steel whose side length of a cross section perpendicular to the longitudinal direction is 250 mm or more is used.

The cast steel is produced as a cast steel having a rectangular cross section by continuous casting or ingot casting. If the side length of the rectangular cross section is less than 250 mm, sulfide particles increase in size in the solidification of the cast steel. Consequently, coarse sulfides remain even after the cast steel is subsequently rolled to obtain a billet, which is disadvantageous in terms of sulfide refinement after final hot working. The side length of the cast steel in the cross section is therefore 250 mm or more. The side length of the cast steel in the cross section is more preferably 300 mm or more. Although no upper limit is placed on the side length of the cast steel in the cross section, the side length is preferably 600 mm or less from the viewpoint of the rollability in the hot rolling following the casting.

(Hot Rolling of Cast Steel into Billet)

Heating Temperature of Cast Steel: 1120° C. or More

The cast steel is hot rolled into a billet. The heating temperature in the hot rolling needs to be 1120° C. or more. If the heating temperature is less than 1120° C., coarse sulfides crystallized during cooling-solidification in the casting stage do not dissolve, and remain even in the billet. Consequently, the sulfides remain coarse even after the hot working, and the desired fine sulfide distribution state cannot be achieved. Accordingly, the heating temperature when hot rolling the cast steel into the billet is 1120° C. or more, and is preferably 1150° C. or more. Although no upper limit is placed on the heating temperature of the cast steel, the heating temperature is preferably 1300° C. or less and more preferably 1250° C. or less from the viewpoint of preventing scale loss.

Area Reduction Rate in Hot Rolling of Cast Steel into Billet: 60% or More

Since the sulfide particles crystallized during the solidification are large in size, the sulfide particles need to be reduced in size to some extent in bloom rolling. If the area reduction rate in the hot rolling is low, the sulfide particles remain large in the billet. In such a case, it is difficult to refine the sulfide particles in heating/rolling when subsequently hot working the billet into a steel bar or a wire rod. In view of this, the area reduction rate in the hot rolling of the cast steel into the billet is 60% or more.

The area reduction rate (%) in the hot rolling can be calculated according to the following formula:

100×(S0−S1)/S0

where S0 is the cross-sectional area of a cross section perpendicular to the hot rolling direction of the cast steel before the hot rolling, and S1 is the cross-sectional area of a cross section perpendicular to the hot rolling direction of the billet produced as a result of the hot rolling.

(Hot Working of Billet)

Heating Temperature: 1050° C. or More

The heating temperature when hot working the billet into a steel bar or a wire rod is an important factor. If the heating temperature is less than 1050° C., the sulfides do not disperse finely, so that the lubricating action during cutting work is poor. This facilitates tool wear, and shortens the tool life. The heating temperature of the billet is therefore 1050° C. or more. The heating temperature of the billet is more preferably 1080° C. or more. Although no upper limit is placed on the heating temperature of the billet, the heating temperature is preferably 1250° C. or less from the viewpoint of suppressing a yield rate decrease caused by scale loss.

Area Reduction Rate in Hot Working: 75% or More

The area reduction rate when hot working the billet into a steel bar or a wire rod is also an important factor for sulfide refinement. If the area reduction rate is less than 75%, sulfide refinement is insufficient. Accordingly, the lower limit of the area reduction rate is 75%. The area reduction rate is more preferably 80% or more. The area reduction rate in the hot working can be calculated according to the following formula:

100×(S1−S2)/S1

where S1 is the cross-sectional area of a cross section perpendicular to the hot working direction of the billet before the hot working, and S2 is the cross-sectional area of a cross section perpendicular to the hot working direction (stretching direction) of the steel bar or wire rod produced as a result of the hot working.

By limiting the size and the heating temperature of the bloom, the size and the heating temperature of the billet, and the area reduction rates to the respective appropriate ranges, the sulfides can be refined and the machinability by cutting can be improved.

Examples

The presently disclosed technique will be described in detail below by way of examples.

Steels having the chemical compositions listed in Table 1 were cast into rectangular cast steels having the dimensions listed in Table 2-1 and Table 2-2 in a cross section perpendicular to the longitudinal direction, by a continuous casting machine. The obtained cast steels were rolled into steel bars under the production conditions listed in Table 2-1 and Table 2-2. Disclosed steels (conforming steels) and comparative steels were subjected to the following test. In detail, the cast steels were each hot rolled at the corresponding heating temperature and area reduction rate in Table 2-1 and Table 2-2, to obtain a square billet having the corresponding long side dimension and short side dimension in Table 2-1 and Table 2-2. The obtained billet was heated at the corresponding heating temperature in Table 2-1 and Table 2-2, and hot rolled into a steel bar having the corresponding diameter in Table 2-1 and Table 2-2. Each of the obtained steel bars (disclosed steels and comparative steels) was subjected to the following test.

TABLE 1 (mass %) No. C Si Mn P S Cr Al Sb N O Others A value* Category  1 0.05 — 0.67 0.072 0.412 0.80 0.001 0.0010 0.0110 0.0295 — 11.0 Conforming Example  2 0.06 0.05 0.55 0.036 0.450 0.55 0.003 0.0040 0.0095 0.0245 — 6.1 Conforming Example  3 0.08 0.02 1.25 0.065 0.356 1.25 0.002 0.0040 0.0123 0.0159 — 17.6 Conforming Example  4 0.03 0.09 0.75 0.051 0.255 0.86 0.001 — 0.0105 0.0163 — 16.0 Conforming Example  5 0.04 0.01 0.83 0.049 0.523 0.98 0.001 — 0.0088 0.0204 — 8.8 Conforming Example  6 0.08 — 1.44 0.007 0.375 0.99 0.002 0.0010 0.0090 0.0288 Ca: 0.0005 15.6 Conforming Example  7 0.05 — 0.86 0.055 0.406 1.23 0.002 — 0.0086 0.0369 Se: 0.12 13.3 Conforming Example  8 0.07 0.02 0.55 0.082 0.324 0.76 0.002 — 0.0120 0.0234 Te: 0.15 10.4 Conforming Example  9 0.06 0.06 1.45 0.091 0.554 1.16 0.002 — 0.0099 0.0254 Bi: 0.05, Sn: 0.010 11.5 Conforming Example 10 0.04 0.01 0.92 0.081 0.543 1.15 0.001 0.0102 0.0060 0.0265 Sb: 0.045 9.7 Conforming Example 11 0.07 0.02 1.05 0.065 0.368 1.15 0.002 — 0.0063 0.0316 B: 0.0035 15.1 Conforming Example 12 0.07 0.02 0.78 0.078 0.435 0.97 0.001 — 0.0077 0.0203 C: 0.25, Ni: 0.15 10.3 Conforming Example 13 0.06 0.03 1.44 0.075 0.366 1.11 0.001 — 0.0096 0.0314 TiO: 0.056 17.0 Conforming Example 14 0.05 — 0.76 0.068 0.370 0.54 0.003 0.0006 0.0089 0.0163 V: 0.008, Zr: 0.06 8.5 Conforming Example 15 0.06 0.02 1.24 0.074 0.399 1.23 0.001 0.0006 0.0123 0.0234 Mg: 0.0009 15.5 Conforming Example 16 0.05 0.01 0.78 0.008 0.399 0.99 0.003 0.0068 0.0040 0.0132 — 11.4 Conforming Example 17 0.09 0.01 0.85 0.055 0.403 0.95 0.003 0.0025 0.0088 0.0126 — 11.3 Comparative Example 18 0.08 0.51 1.15 0.016 0.435 0.88 0.001 0.0025 0.0123 0.0168 — 11.4 Comparative Example 19 0.08 0.02 0.45 0.045 0.352 0.56 0.002 0.0036 0.0098 0.0201 — 7.3 Comparative Example 20 0.05 0.01 2.13 0.060 0.301 0.55 0.003 0.0056 0.0076 0.0176 — 19.6 Comparative Example 21 0.09 — 0.84 0.120 0.406 0.25 0.001 0.0019 0.0089 0.0155 — 6.0 Comparative Example 22 0.08 0.01 0.75 0.096 0.241 0.65 0.001 0.0019 0.0112 0.0201 — 14.3 Comparative Example 23 0.07 0.02 0.53 0.012 0.611 1.09 0.002 0.0019 0.0098 0.0196 — 7.1 Comparative Example 24 0.05 0.01 1.36 0.003 0.352 0.04 0.003 0.0019 0.0053 0.0162 — 8.1 Comparative Example 25 0.07 0.02 0.94 0.065 0.463 1.59 0.001 0.0019 0.0123 0.0246 — 14.4 Comparative Example 26 0.05 0.01 1.00 0.013 0.349 1.06 0.013 — 0.0062 0.0222 — 14.8 Comparative Example 27 0.06 — 1.34 0.065 0.391 1.25 0.003 0.0068 0.0170 0.0116 — 16.4 Comparative Example 28 0.05 0.53 0.63 0.023 0.406 0.95 0.001 0.0088 0.0123 0.0091 — 10.1 Comparative Example 29 0.07 0.03 1.36 0.098 0.369 0.95 0.003 0.0088 0.0076 0.0523 — 15.1 Comparative Example 30 0.08 0.02 0.68 0.023 0.531 0.57 0.001 0.0088 0.0116 0.0165 — 5.8 Comparative Example 31 0.04 — 1.25 0.089 0.312 1.08 0.004 0.0088 0.0084 0.0203 — 18.4 Comparative Example *Avalue = 2(Mn + 2Cr)/S ratio: conforming range (6.0 to 18.0). “—” in composition table denotes less than 0.01 for Si, and less than 0.003 for Sb.

A test piece was collected from a cross section parallel to the rolling direction of the obtained steel bar, and the ¼ position in the radial direction from the peripheral surface of the cross section was observed with a scanning electron microscope (SEM) to investigate the equivalent circle diameter and number density of sulfide in the steel. Here, precipitate composition analysis was conducted by energy dispersive X-ray spectrometry (EDX). The obtained SEM images of precipitates determined as sulfide by EDX were analyzed and binarized to calculate the equivalent circle diameter and the number density.

The machinability by cutting was evaluated by an outer periphery turning test. BNC-34C5 produced by Citizen Machinery Co., Ltd. was used as a cutting machine, Carbide EX35 Tool TNGG160404R-N produced by Hitachi Tool Engineering, Ltd. was used as a turning tip, and DTGNR2020 produced by KYOCERA Corporation was used as a holder. As a lubricant, a 15-fold diluted emulsion of YUSHIROKEN FGE1010 produced by Yushiro Chemical Industry Co., Ltd. was used. The cutting conditions were cutting rate: 120 m/min, feed rate: 0.05 mm/rev, cut depth: 2.0 mm, and machining length: 10 m.

The machinability by cutting was evaluated based on the flank wear Vb of the tool after the end of the cutting test over a length of 10 m. In the case where the flank wear Vb after the end of the cutting test was 200 μm or less, the machinability by cutting was evaluated as “good”. In the case where the flank wear was more than 200 μm, the machinability by cutting was evaluated as “poor”.

The test results of the disclosed steels and the comparative steels are shown in Table 2-1 and Table 2-2. As is clear from Table 2-1 and Table 2-2, the disclosed steels had favorable machinability by cutting as compared with the comparative steels.

TABLE 2-1 Properties of steelbar (inclusion distribution, machinability by cutting test result) Number Cast steel rolling (rolling cast steel into billet) Linear rod rolling (rolling billet into steelbar) density of Number Long Short Area sulfides of density of side of side of Area Long Short reduction less than sulfides of cross cross reduction side of side of rate in 1 μm in 1 to 5 μm in section section Cross- rate in cross cross Cross- linear equivalent equivalent Steel of cast of cast sectional Heating cast steel section section sectional Heating Steel bar rod circle circle Tool life sample steel steel area temperature rolling of billet of billet area temperature diameter rolling diameter diameter (machinability No. No. (mm) (mm) (mm²) (° C.) (%) (mm) (mm) (mm²) (° C.) (mm) (%) (particles/mm²) (particles/mm²) by cutting) Remarks  1  1 400 300 120000 1180 79 160 160 25600 1080 25 98 1273 2896 Good Example  2  2 400 300 120000 1180 79 160 160 25600 1080 25 98 1011 2299 Good Example  3  3 400 300 120000 1180 79 160 160 25600 1080 25 98 1817 4134 Good Example  4  4 400 300 120000 1180 79 160 160 25600 1080 25 98  810 2343 Good Example  5  5 400 300 120000 1180 79 160 160 25600 1080 25 98 1986 4518 Good Example  6  6 400 300 120000 1180 79 160 160 25600 1080 25 98 1746 3971 Good Example  7  7 400 300 120000 1180 79 160 160 25600 1080 25 98 1835 4174 Good Example  8  8 400 300 120000 1180 79 160 160 25600 1080 25 98  913 2077 Good Example  9  9 400 300 120000 1180 79 160 160 25600 1080 25 98 2843 6467 Good Example 10 10 400 300 120000 1180 79 160 160 25600 1080 25 98 2380 5414 Good Example 11 11 400 300 120000 1180 79 160 160 25600 1080 25 98 1678 3817 Good Example 12 12 400 300 120000 1180 79 160 160 25600 1080 25 98 1611 3664 Good Example 13 13 400 300 120000 1180 79 160 160 25600 1080 25 98 1823 4148 Good Example 14 14 400 300 120000 1180 79 160 160 25600 1080 25 98  927 2108 Good Example 15 15 400 300 120000 1180 79 160 160 25600 1080 25 98 2009 4571 Good Example 16  1 420 350 147000 1180 83 160 160 25600 1080 25 98 1444 3001 Good Example 17  1 400 300 120000 1220 79 160 160 25600 1080 25 98 1564 3265 Good Example 18  1 400 300 120000 1180 84 140 140 19600 1080 25 97 1654 3269 Good Example 19  1 400 300 120000 1180 84 140 140 19600 1130 25 97 1312 2130 Good Example 20  1 400 300 120000 1180 84 140 140 19600 1080 15 99 1273 3356 Good Example 21  4 250 250  62500 1120 60 158 158 24964 1050 89 75  511 2021 Good Example 22 16 400 300 120000 1180 79 160 160 25600 1080 30 97 1124 2558 Good Example *1 Underlines indicate outside applicable range. *2 Number density of sulfides of less than l μm in equivalent circle diameter: conforming range (at least 500 particles/mm²). *3 Number density of sulfides of 1 to 5 μm in equivalent circle diameter: conforming range (at least 2000 particles/mm²). *4 Tool life (machinability by cutting) good: tool wear of 200 μm or less, poor: tool wear of more than 200 μm.

TABLE 2-2 Linear rod rolling Cast steel rolling (rolling cast steel into billet) (rolling billet into steel bar) Long Short Long side of side of Area side of Short cross cross reduction cross side of section section Cross- rate in section cross Cross- Steel of cast of cast sectional Heating cast steel of section sectional sample steel steel area temperature rolling billet of billet area No. No. (mm) (mm) (mm²) (° C.) (%) (mm) (mm) (mm²) 23  1 257 240  61680 1120 60 158 158 24964 24  1 230 230  52900 1180 52 160 160 25600 25  1 400 300 120000 1100 79 160 160 25600 26  1 250 250  62500 1180 59 160 160 25600 27  1 400 300 120000 1180 79 160 160 25600 28  1 400 300 120000 1180 79 160 160 25600 29 17 400 300 120000 1180 79 160 160 25600 30 18 400 300 120000 1180 79 160 160 25600 31 19 400 300 120000 1180 79 160 160 25600 32 20 400 300 120000 1180 79 160 160 25600 33 21 400 300 120000 1180 79 160 160 25600 34 22 400 300 120000 1180 79 160 160 25600 35 23 400 300 120000 1180 79 160 160 25600 36 24 400 300 120000 1180 79 160 160 25600 37 25 400 300 120000 1180 79 160 160 25600 38 26 400 300 120000 1180 79 160 160 25600 39 27 400 300 120000 1180 79 160 160 25600 40 28 400 300 120000 1180 79 160 160 25600 41 29 400 300 120000 1180 79 160 160 25600 42 30 400 300 120000 1180 79 160 160 25600 43 31 400 300 120000 1180 79 160 160 25600 Properties of steel bar (inclusion distribution, machinability by cutting test result) Linear rod rolling Number (rolling billet into steel bar) density of Number Area sulfides of density of reduction less than sulfides of rate in 1 μm in 1 to 5 μm in linear equivalent equivalent Heating Steel bar rod circle circle Tool life temperature diameter rolling diameter diameter (machinability No. (° C.) (mm) (%) (particles/mm²) (particles/mm²) by cutting) Remarks 23 1050 89 75  483 2034 Poor Comparative Example 24 1080 25 98  324 1804 Poor Comparative Example 25 1080 25 98  514 1589 Poor Comparative Example 26 1080 25 98  569 1756 Poor Comparative Example 27 1030 25 98 1023 1465 Poor Comparative Example 28 1080 95 72  468 1786 Poor Comparative Example 29 1080 30 97 1131 2574 Poor Comparative Example 30 1080 30 97 1292 2940 Poor Comparative Example 31 1080 30 97  456 1114 Poor Comparative Example 32 1080 30 97  356 1375 Poor Comparative Example 33 1080 30 97  756 2146 Poor Comparative Example 34 1080 30 97  467 1805 Poor Comparative Example 35 1080 30 97  444 1769 Poor Comparative Example 36 1080 30 97  324 1657 Poor Comparative Example 37 1080 30 97 1156 2146 Poor Comparative Example 38 1080 30 97 1112 2529 Poor Comparative Example 39 1080 30 97 1533 3487 Poor Comparative Example 40 1080 30 97 1154 2179 Poor Comparative Example 41 1080 30 97 1232 2217 Poor Comparative Example 42 1080 30 97  430 1567 Poor Comparative Example 43 1080 30 97  398 1765 Poor Comparative Example *1 Underlines indicate outside applicable range. *2 Number density of sulfides of less than 1 μm in equivalent circle diameter: conforming range (at least 500 particles/mm²). *3 Number density of sulfides of 1 to 5 μm in equivalent circle diameter: conforming range (at least 2000 particles/mm²). *4 Tool life (machinability by cutting) good: tool wear of 200 μm or less, poor: tool wear of more than 200 μm 

1. A free-cutting steel comprising: a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.0100% and 0.0500% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0, A value=2([Mn]+2[Cr])/[S]  (1) where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S; and a steel microstructure in which at least 500 particles/mm² of sulfide of less than 1 μm in equivalent circle diameter and at least 2000 particles/mm² of sulfide of 1 μm to 5 μm in equivalent circle diameter are distributed.
 2. The free-cutting steel according to claim 1, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Si: 0.50% or less, P: 0.10% or less, Al: 0.010% or less, and N: 0.0150% or less.
 3. The free-cutting steel according to claim 1, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
 4. A method of producing a free-cutting steel, the method comprising: rolling a rectangular cast steel at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, the rectangular cast steel having a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.010% and 0.050% or less, and Cr: 0.50% to 1.50% with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0, A value=2([Mn]+2[Cr])/[S]  (1) where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S, and a side length of a cross section of the rectangular cast steel perpendicular to a longitudinal direction being 250 mm or more; and hot working the billet at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.
 5. The method of producing a free-cutting steel according to claim 4, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Si: 0.50% or less, P: 0.10% or less, Al: 0.010% or less, and N: 0.0150% or less.
 6. The method of producing a free-cutting steel according to claim 4, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
 7. The free-cutting steel according to claim 2, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less.
 8. The method of producing a free-cutting steel according to claim 5, wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of Ca: 0.0010% or less, Se: 0.30% or less, Te: 0.15% or less, Bi: 0.20% or less, Sn: 0.020% or less, Sb: 0.025% or less, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ti: 0.100% or less, V: 0.20% or less, Zr: 0.050% or less, and Mg: 0.0050% or less. 